An electric power steering apparatus which provides a steering mechanism of a vehicle with a steering assist torque (an assist torque) by means of a rotational torque of a motor, applies a driving force of the motor as the assist torque to a steering shaft or a rack shaft by means of a transmission mechanism such as gears or a belt through a reduction mechanism. In order to accurately generate the assist torque, such a conventional electric power steering apparatus performs a feedback control of a motor current. The feedback control adjusts a voltage supplied to the motor so that a difference between a current command value and a motor current becomes small, and the adjustment of the voltage applied to the motor is generally performed by an adjustment of a duty ratio of a PWM (Pulse Width Modulation) control.
A general configuration of such an electric power steering apparatus will be described with reference to FIG. 1. A column shaft 2 connected to a steering wheel (handle) 1 is connected to tie rods 6 of steered wheels through reduction gears 3, universal joints 4A and 4B, and a rack and pinion mechanism 5. The column shaft 2 is provided with a torque sensor 10 for detecting the steering torque of the steering wheel 1 in accordance with torsion of a torsion bar, and a motor 20 for assisting the steering force of the steering wheel 1 is connected to the column shaft 2 through the reduction gears 3. Electric power is supplied to a control unit 30 for controlling the electric power steering apparatus from a battery 14, and an ignition key signal is inputted into the control unit 100 through an ignition key 11. The control unit 30 calculates a current command value I of an assist command based on a steering torque Tr detected by the torque sensor 10 and a velocity Vel detected by a velocity sensor 12, and controls a current supplied to the motor 20 based on the calculated current command value I.
The control unit 30 mainly comprises a CPU (or an MPU or an MCU), and general functions performed by a program within the CPU are shown in FIG. 2.
The functions and operations of the control unit 30 will be described with reference to FIG. 2. The steering torque Tr detected by the torque sensor 10 and the velocity Vel from the velocity sensor 12 are inputted to a steering assist command value calculating section 31, and a steering assist command value Iref is calculated by using an assist map. With respect to the calculated steering assist command value Iref, the output is limited based on an overheat protection condition in a maximum output limiting section 32, the current command value I that the maximum output is limited is inputted to a subtracting section 33.
Moreover, in addition to the steering torque Tr and the velocity Vel, it is also possible to calculate the steering assist command value Iref by using a steering angle in the steering assist command value calculating section 31.
The subtracting section 33 obtains a deviation ΔI(=I−i) between the current command value I and a motor current i of the motor 20 that is fed back, the deviation ΔI is controlled by a current control section 34 such as PI (proportional and integral), the controlled current control value E is inputted to a PWM-control section 35 to calculate a duty ratio, and drives the motor 20 through a motor driving circuit 36. The motor current i of the motor 20 is detected by a motor current detecting circuit 37, and the motor current i is inputted to the subtracting section 33 to be fed back.
A configuration example of the motor driving circuit 36 will be described with reference to FIG. 3. In the case of a three-phase motor, the motor driving circuit 36 comprises an FET gate driving circuit 361 that drives each of field-effect transistors (FET1 to FET6) based on a pulse width modulation (PWM) signal from the PWM control section 35, an inverter 362 comprising a three-phase bridge circuit of FET1 to FET6 and a step-up power supply 363 that drives high side FETs (FET1, FET2 and FET3). Moreover, with respect to FET to FET6, a diode for surge absorbing is connected in anti-parallel between source and drain, respectively. Electric power is supplied from the battery 14 as a power supply to the inverter 362 through the ignition key 11 and a power relay RL. The inverter 362 comprises an FET-array that an FET1 and an FET4 connected in series, an FET-array that an FET2 and an FET5 connected in series, and an FET-array that an FET3 and an FET6 connected in series, these three FET-arrays connected in series, are connected in parallel. From a connecting point of the FET1 and the FET4 in the inverter 362, a connecting point of the FET2 and the FET5 in the inverter 362 and a connecting point of the FET3 and the FET6 in the inverter 362, each motor phase current is supplied to the motor 20 through supply routes “a”, “b” and “c”.
In such an electric power steering apparatus, the battery 14 supplies the electric power to loading apparatuses such as the control unit 30, the torque sensor 10, the motor 20 and so on. In order to assist for steering operations of a driver to be stable normally, it is necessary to maintain the power voltage of the battery 14 in a given stable range (for example, 10V-15V). However, in a situation such as cranking, there is a possibility that the power voltage reduction occurs.
In a state that the power voltage dropped, the gate driving voltage of the FET used in the motor driving circuit 36 drops. In this case, when the voltage (VGS) from gate to source of the FET dropped, the drain-source on-state resistance (RDS (ON)) becomes large abruptly. For comparison, there is a relation such as the following Expression 1 between a maximum driving current Imax and an allowable power value P of the FET.P=RDS(ON)·Imax2  [Expression 1]                where, “P” is the allowable power value of the FET, “RDS (ON)” is the drain-source on-state resistance of the FET, and “Imax” is a motor maximum current that can pass in the FET.        
From the relation of the above Expression 1, in the case of drive control of the motor 20, when the drain-source on-state resistance (RDS (ON)) of the FET becomes large, power loss also becomes large. Therefore, when the power voltage dropped, due to heat occurred by power loss of the FETs, the temperature increases. In addition, when the power voltage reduction continues, there is a possibility that a failure that the FET is damaged by burnout occurs.
Further, when the power voltage dropped dramatically and dropped to less than or equal to a sensor minimum operating voltage of the torque sensor 10, the output of the torque sensor 10 descends, a neutral position of the steering wheel 1 becomes being off track, the current characteristic of the motor 20 also becomes being off track from the neutral position of the steering wheel 1. Therefore, there was such a problem, that is, a bilateral difference of the steering force of the steering wheel occurs, when the bilateral difference becomes abysmal, a trouble such as “the steering wheel is taken” occurs, the steering feeling becomes bad. That is to say, when the power voltage becomes less than or equal to a certain voltage value, the torque sensor 10 cannot work normally.
Therefore, when the power voltage dropped, in order to keep a good steering feeling, it is necessary to limit or shut down the assist control. In order to solve such a problem, in Patent Document 1 (Japanese Patent Application Laid-open No. 2005-193751), an electric power steering apparatus that limits the assist amount by the means of a variable limitation value in accordance with the power voltage in the case of the power voltage drop, is proposed. Further, in Patent Document 2 (Japanese Patent Application Laid-open No. 2007-290429), an electric power steering apparatus that comprises semiconductor switching elements with a low on-state resistance at a time of low voltage, when the power voltage is more than or equal to a lower limit of the operation voltage, performs the control of the electric motor, and when the power voltage is less than the operation voltage, shuts down the control of the electric motor, is proposed.